Summary Notes. The Atmosphere

Summary notes The Atmosphere Atmosphere important gases in Earth’s atmosphere – N2 = 78%, O2 = 21%, Ar = 0.9% + H2O, CO2, O3 CO2 increase as a result of burning fossil fuels, shows seasonal variation O3 produced and destroyed in stratosphere air pressure falls off with altitude (halves about every 5.5 km) caused by weight of air above temperature structure of atmosphere names of layers (troposphere thru’ thermosphere) and boundaries separating them approximate altitude of tropopause why does temperature fall off with altitude in troposphere why does temperature increase in stratosphere and thermosphere in which layer does all weather, winds, rain etc. happen Energy and Radiation mechanisms of heat transfer conduction = transfer of heat through matter via collisions between molecules (requires contact. air is poor conductor. metals are good conductors.) convection = transfer of heat by mass movement (can only take place in fluids, gases) – responsible for rising parcels of warm air, descending parcels of cool air; and for global circulation radiation = heat transfer via electromagnetic waves (light) can travel through a vacuum electromagnetic spectrum radiation = light (some is visible to us, some is not) photon = packet of light energy, travels at speed of light the electromagnetic spectrum – in order of increasing wavelength (= decreasing energy): X-rays, Ultraviolet light, visible light, infrared, microwave, radio waves everything gives off radiation. The hotter the object the higher the energy of the radiation given off, and the longer the wavelength. The Sun gives off most energy in the visible part of the spectrum. the wavelengths of radiation emitted by an object depend on the object’s temperature - the hotter the object, the shorter the wavelength of radiation emitted The earth absorbs nearly all the solar radiation which arrives at its surface and emits the energy at infrared wavelengths The gases that absorb in the infrared spectrum block outgoing radiation from the earth and re-radiate it back to the earth’s surface, warming the surface. These are Greenhouse gases. Energy balance: albedo of earth-atmosphere ~ 30% (20% reflected by clouds, 10% by earth’s surface and scattered back by gases) ~19% of incoming solar radiation is absorbed by gases, dust and clouds in the atmosphere ~51% absorbed at the earth’s surface Most of the radiation emitted by the earth is absorbed in the atmosphere by greenhouse gases and clouds. Some of this energy will be re-radiated back to the surface of the earth. This raises the temperature of the earth to an equilibrium value of ~ 15 ºC. The greenhouse effect is a natural phenomenon which is necessary to sustain life on earth (otherwise it would be too cold). The reason that we are concerned with rising CO2 levels is that we are enhancing the natural greenhouse effect and thus warming up the earth. If the earth warms up by even 1 or 2 degrees our climate will change dramatically and ice caps could melt, sea levels rise, and coastal cities flood. Temperature Effect of land and water on temperature Land heats more rapidly and to higher temperatures than water, and cools more rapidly and to lower temperatures than water, so temperature variations over land are very much greater than over water. Winds are from the west, so west coast has strong ocean influence. The middle of continents and E coast are not influenced much by the ocean. Why are temperature changes much smaller over ocean? 1. specific heat of water compared to land 2. water is highly mobile 3. water is more transparent 4. evaporation greater over water (some heat is used as latent heat) Compare annual temperature variations of coastal and inland cities Humidity and Saturation vapor pressure (v.p.) = pressure resulting from water vapor molecules in air (mb) dew point = temperature at which the water vapor in the air is saturated If air is unsaturated there are 2 ways of bringing it to saturation: 1. cool it (keeping the amount of water vapor the same) 2. add water vapor to it (keeping the temperature the same) Low dew point = dry air High dew point = moist air Condensation and Clouds air temperature must be at or below dew point for condensation to occur when air rises it passes through regions of successively lower pressure, expands and cools. It cools at a rate of about 10°C/km. If air rises enough it will cool to the dew point and condensation can occur. The height at which this occurs is the lifting condensation level. Processes that force air to rise: convective lifting: local surface heating, rising warm air, cool air sinks forming convective cell circulation orographic lifting: horizontally moving air forced to rise over a mountain (topographic barrier) frontal wedging: formed when masses of warm and cold air meet. warmer, less dense air forced to rise over colder denser air. fog = cloud on the ground atmosphere is stable if it inhibits rising (ie. air parcel is colder than the environment) atmosphere is unstable if it allows rising (ie. air parcel is warmer than the environment) Precipitation typical cloud droplets are too small to produce rain. droplet growth by vapor condensation is too slow to produce a raindrop (would take a day or more) cloud droplets grow to form raindrops by the process of collision and coalescence (or coagulation): ice can form in clouds if the air is cold enough. Ice crystals can fall as snow or rain – it depends on the temperature at which the crystal forms and the temperature below the cloud (i.e. whether the ice melts on its way to the ground) cloud classification: cirrus – high, white, thin cumulus – globular individual cloud masses stratus – sheets or layers. no distinct cloud units. Air Pressure pressure is exerted by weight of air above halves about every 5.5 km lines of constant pressure are isobars surface pressure at earth is around 1000 mb ‘H’ = center of high pressure = anticyclone ‘L’ = center of low pressure = cyclone Winds wind is caused by horizontal differences in pressure – which are caused by unequal heating if earth did not rotate and there was no friction, air would go directly from high -> low pressure pressure gradient force: spacing of isobars shows pressure gradient = difference in pressure/distance. The closer the isobars the greater the pressure gradient, and the greater the pressure gradient force, and the stronger the wind. pressure gradient force is perpendicular to isobars – in a direction from high to low pressure. Coriolis force: apparent force due to rotation of earth (if we were fixed out in space we would not see this force. It is ‘apparent’ because it exists only because we are on a rotating earth and so to us free-moving objects are deflected from straight-line travel.) in NH objects (winds) are deflected to the right in SH objects (winds) are deflected to the left CF – is at right angles to direction of movement strongest at poles, 0 at equator Geostrophic wind = wind generated by balance between PGF and CF. flows parallel along straight isobars stronger for closely-spaced isobars For a Low, flow is anticlockwise in NH, clockwise in SH For a High, flow is clockwise in NH, anticlockwise in SH Surface wind = wind at/close to earth’s surface where friction is important friction caused by trees, buildings, topography PGF directed from H to L CF at right angles to wind friction directly opposes wind causes wind to blow at an angle to isobars (in towards L, out away from H) Vertical air motion surface lows (cyclones) cause air to blow inwards = convergence -> air must rise > divergence aloft. Therefore surface lows are associated with rising air, clouds, precipitation. surface highs (anticyclones) cause air to blow outwards = divergence at surface -> convergence aloft -> subsidence of air. Therefore surface highs are associated with sinking (and warming) air and fair weather. Global circulation 3-cell model Hadley cell: equator - 30° - rising air at equator Equatorial low, 30° high = Subtropical High, 60° low, 90° polar high Surface winds –Trade winds, Westerlies, Polar front, Easterlies Zones of precipitation wet at equatorial low dry in subtropical high wet at subpolar low dry at polar high zones shift with season actual precipitation patterns affected by distribution of land and water ITCZ = Intertropical convergence zone (where winds come together from 2 hemispheres) Fronts boundary between air of two different temperatures (and density) get cold fronts, warm fronts, (and occluded fronts) Learn weather map symbols (wind directions and speed, symbols for fronts, pressures)